/Engine/Plugins/Editor/UVEditor/Source/UVEditor/Public/SUVEditor2DViewport.h
These allow the toolkit to add an accept/cancel overlay when needed. PopulateViewportOverlays is not helpful here because that gets called just once.
virtual void AddOverlayWidget ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > OverlaidWidget )